The present invention generally relates to semiconductor photodiodes, and more particularly to an integrated circuit device that includes photodiodes, and a process for its fabrication.
The structure and function of semiconductor photodiodes are well known in the art Photodiodes convert electromagnetic radiation in the form of photons into electrical energy. Typical photodiodes operate in the visible and near-infrared range of the electromagnetic radiation spectrum. Different semiconductor materials determine the particular wavelength of the radiation to which the photodiode responds. Photodiodes can be fabricated from elemental semiconductors, such as germanium and silicon, as well as so-called III-V compound semiconductors, such as gallium-arsenide.
A typical photodiode includes a surface P-type anode region to which an anode contact is formed. An antireflective film overlies the P-type region and is structured to assure a high degree of transmission of radiation at the wavelength that the photodiode is designed to absorb. Beneath the P-type region is a very lightly-doped N-type drift region in which photons of the incident radiation are absorbed, generating hole-electron pairs. Adjoining the N-type drift region is a heavily-doped N+ cathode region, to which a cathode contact is formed at a surface of the device. The PN junction between the P-type anode region and the N-type drift region is reverse biased by an applied potential producing a depletion layer on both sides of the junction. Because the N-type drift region is relatively lightly doped, the depletion layer predominantly resides on the N-type side of the junction extending deeply into the drift region. Holes and elections generated in the depletion layer are swept in opposite directions in response to the applied potential, providing a current that is a function of the incident radiation.
It is desirable for certain applications to include more than one photodiode as part of an integrated circuit. U.S. Pat. No. 5,177,581 is an example of a prior-art patent describing the incorporation of multiple identical photodiodes on an integrated circuit chip.
It is known that trade-offs in the design of photodiodes determine various operational characteristics. Also, when integrating a photodiode on the same semiconductor chip as other elements (transistors, resistors, etc.) to perform complex functions in response in part to incident radiation signals, the constraints of the process for making such other elements must be considered in the design of the photodiode. It is desirable to minimize the complexity of a semiconductor fabrication process while maximizing the flexibility available to the designer to provide complex functionality in the device design. The inclusion of a photodiode on an integrated circuit chip made with state-of-the-art CMOS or BiCMOS process technology contributes to the foregoing design considerations. CMOS devices include complementary types of MOS transistors (both PMOS and NMOS). BiCMOS devices include MOS transistors as well as bipolar transistors.
In accordance with a principal object of the invention, photodiodes of different structures are integrated on a semiconductor chip with transistors defining an integrated circuit. The photodiodes are built up using structures that correspond to structures of the transistors. Thus, a process for making a CMOS integrated circuit device can be adapted without altering the process steps by using different photomasks to incorporate the photodiodes along with PMOS and NMOS transistors into a single optoelectronic integrated circuit device.
Both fast and efficient types of photodiodes can be integrated on the same chip using anode regions of different depths, the anode regions corresponding to PLDD regions of PMOS transistors or to P-wells of NMOS transistors. An antireflective film used over the silicon surface of the photodiodes also functions as a silicide-blocking mask at other locations on the device.